Method for removing fumigation gases from fumigated structures

ABSTRACT

A method for removing fumigation gasses from fumigated structures is provided. Air is directed from a structure through an adsorption unit and then recirculated back into the structure. A fan directs air through the adsorption unit. The adsorption unit contains an annularly shaped adsorption bed and an inner passage way and outer passage way through which fumigation gasses are directed. A lower adsorption efficiency per pass through the adsorption bed is used in conjunction with multiple passes with the method of the present invention.

BACKGROUND OF THE INVENTION

The present invention relates to the fumigation of agriculturalproducts. More particularly, an apparatus and method are provided forremoving fumigation gasses from a structure.

Various fumigation methods are used to control pests in agriculturalproducts. In general, a fumigation agent is placed in an agriculturalstorage structure in order to control any pests present. For example,widely used fumigation agents include aluminum phosphide and magnesiumphosphide (both of which release hydrogen phosphide gas). Methyl bromideis also widely used as a fumigation agent. The pests to be controlledare usually insects such as the lesser grain borer and the granaryweevil, but also may include rodents. Numerous agricultural productssuch as grains, rice, beans, and tobacco may be fumigated. Thefumigation may occur in a variety of different agriculture storage ortransportation structures or facilities including, for example, silos,bins, warehouses, caverns, railroad cars, barges or ships.

Prior to a fumigation, the structure is generally sealed in a gas tightmanner to minimize the escape of the fumigation gasses into theatmosphere. In performing a fumigation, a fumigation agent may beintroduced to the storage structure in the form of a liquid, solid orgas. Gasiform fumigants released from the fumigation agent then travelthroughout the agricultural product and kill the pests. In order touniformly spread the fumigation gasses, the agricultural product may bemixed as the dose is applied, the fumigation agent may be probed intothe product, or the air within the structure may be circulated. Forexample, U.S. Pat. No. 4,200,657 to Cook discloses a method ofrecirculating the air within a structure in order to aid the fumigationgas distribution.

Following an appropriate fumigation exposure time, the structure isusually ventilated to remove the fumigation gasses. Traditionally astructure was exhausted by simply opening the structure to allow thegases to escape or be blown into the atmosphere. However, as safety andenvironmental concerns increased, there arose a need to preventfumigation gasses from escaping into the atmosphere, and thus methods of"scrubbing" or removing fumigation gasses from exhaust gas streams havebeen developed. For example, U.S. Pat. No. 4,966,755 to Smith disclosesa method of removing fumigation gases from an enclosure. In that method,the gas is pumped through an absorbent chamber that removes anyhazardous material prior to exhausting the gas to the atmosphere.Likewise, U.S. Pat. No. 4,812,291 to Friemel et al. discloses a methodof removing hydrogen phosphide from a storage structure afterfumigation. In Friemel, air from a fumigated structure is blown by a fanthrough beds containing a hydrogen phosphide binding and decompositioncatalyst. Upon passing through the beds, the air is exhausted to theatmosphere.

Fumigation scrubbing methods such as disclosed by Smith and Friemel etal. generally entail a single pass of the air through an adsorbent bedprior to exhausting the air to the atmosphere. Thus, such systems removesubstantially all of the fumigation gas in a single pass throughadsorption beds and such methods generally use a large amount ofadsorbent material and large fans or pumps for each adsorption unit.Thus, the costs of each adsorption unit may be high and the size large.It is desirable to minimize both the costs and size and to create aportable system.

SUMMARY OF THE INVENTION

The present invention in a broad aspect comprises a method and apparatusfor removing a gasiform fumigant from an agricultural storage structureby utilizing recirculation. A sorption unit that contains a sorbentmaterial is provided. Fumigant from the structure is forced through thesorption unit and then an output gas from the sorption unit isrecirculated into the structure.

Using the method and apparatus of the present invention, the sorptionunit may have a low efficiency per pass but use multiple passes toachieve a selected level of fumigant removal. Preferably, an adsorbentmaterial is used to remove a fumigant such as hydrogen phosphide. Gasflow rates of less than about 2000 cfm are generally contemplated.

The present invention may be utilized in conjunction with theintroduction of gaseous hydrogen phosphide into an agricultural storagestructure. The hydrogen phosphide may then be removed by using therecirculation method and apparatus described.

It is further contemplated that the adsorption unit of the presentinvention will comprise a first and second port enabling air to flowthrough the unit, an adsorbent bed containing adsorbent material, and afirst and second passageway within the unit. Preferably, the adsorbentbed and one passageway are annularly shaped while the other passagewayis cylindrically shaped. The adsorption unit may be connected to a fanand the storage structure through air ducts.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic, side view of a fumigation gas removal systemaccording to the present invention connected to an agricultural storagestructure.

FIG. 1A is a schematic, side view of an alternative fumigation gasremoval system according to the present invention connected to anagricultural storage structure.

FIG. 2 is an perspective view of an apparatus of the present invention.

FIG. 3 is a cross-sectional view along section line 3--3 of FIG. 2.

FIG. 4 is an end view of the apparatus shown in FIG. 2.

DETAILED DESCRIPTION

Referring to FIG. 1, there is shown an embodiment of the presentinvention. An agricultural storage structure 10, such as a warehouse,silo, barge, ship, railroad car, etc., containing an agriculturalproduct is treated with a fumigating agent. The fumigation agent may beapplied in a variety of methods known to those skilled in the art. Whenit is desired to remove the gasiform fumigant from the air withinstructure 10, blower or fan 12 circulates or purges air from structure10 though an air duct 14 to an adsorption unit 16 and then back intostructure 10 via air duct 18. Air ducts 14 and 18 may be especiallydesigned for fumigation gas removal purposes, or they may be ductsalready installed for other fumigation purposes. Air ducts 14 and 18 mayterminate at edge of structure 10 as shown in FIG. 1 or they may extenda distance into structure 10. Further, air duct 14 may be placed aboveair duct 18 as shown in FIG. 1, or alternatively, may be placed belowair duct 18. Adsorption unit 16 contains an adsorbent material thatremoves the fumigating gas from the air as the air passes through theadsorption unit. According to the present invention, recirculation isthus provided so that air within structure 10 may flow throughadsorption unit 16 in a multi-pass manner during removal of thefumigating gas.

Recirculation provides a variety of advantages over the single-passfumigation gas removal methods shown in the prior art. A single-passmethod requires a substantially 100% adsorption efficiency. That is, asair flows through a single-pass adsorption unit the fumigating gas mustbe removed below an acceptable level in a single-pass. An acceptablelevel of gas removal will depend on the particular application. Forexample, when removing hydrogen phosphide from the air, it may bedesirable to remove more than 96% of the hydrogen phosphide prior toexhausting the air to the atmosphere. The multi-pass method andapparatus of the present invention, though, does not require such a highefficiency per pass. Rather, the system has a lower adsorptionefficiency per pass and uses multiple passes to remove the fumigatingagent below the user's acceptable level. Thus, the amount of adsorbentmaterial required for adsorption unit 16 is less than for a single-passunit. Furthermore, in accordance with the present invention, therecirculation may be carried out over a longer period of time withsmaller fans than generally used with single-pass methods. Thus,reductions in costs and size are available when using the presentinvention.

Alternative embodiments other than the one shown in FIG. 1 may be usedwith the present invention. For example, fan 12 may be placed betweenadsorption unit 16 and air duct 18. In addition, the air flow may becreated in either direction, for example from structure 10 through airduct 18 and back into structure 10 via air duct 14. Alternatively, fan12, adsorption unit 16 and air ducts 14 and 18 may be contained withinstructure 10.

FIG. 1A displays another alternative embodiment. Recirculation isachieved in the system shown in FIG. 1A similar to the system shown inFIG. 1 except air duct 14 is placed at the top of structure 10 and airduct 18 is placed under agricultural product 11. Aeration manifold 19 orother means known in the art aids in distributing the air returning tostructure 10. Recirculation does not have to occur in a top to bottommanner but rather may occur from bottom to top.

Alternatively, air ducts 14 and 18 may draw and return air from the topof structure 10. Finally, depending on the placement of adsorption unit16 or fan 12, it is noted that fan 12 or adsorption unit 16 may connectdirectly with structure 10 rather than through air ducts or even simplybe placed inside structure 10 without using air ducts. Thus variousembodiments of the invention exist; however, in all the embodimentsdiscussed above, multi-pass recirculation through adsorption unit 16 ispossible.

A preferred embodiment of adsorption unit 16 is shown in FIGS. 2-4. Asshown in FIG. 2, adsorption unit 16 is generally cylindrical in shape.FIG. 3 illustrates a cross-sectional view of adsorption unit 16 alongsection line 3 of FIG. 2. FIG. 4 is an end view of adsorption unit 16.As shown more clearly in FIG. 3, adsorption unit 16 has cylindricalshaped air inlet port 30 and cylindrical shaped air outlet port 32.Adsorption unit 16 has cylindrically shaped outer wall 34 with firstendwall 36 adjacent air inlet port 30. Second endwall 38, adjacent airoutlet port 32, is generally funnel shaped. A skid 60 or other supportmeans may be used to support adsorption unit 16. Preferably, adsorptionunit 16 is small enough to be mobile such that the adsorption unit mayreadily service storage facilities of various types and at variouslocations.

Adsorbent bed 40 is supported within adsorption unit 16. Adsorbent bed40 contains adsorbent material 42. Adsorbent bed 40 is generallyannularly shaped and has inner wall 44 and outer wall 46. In onepreferred embodiment, adsorbent bed 40 is approximately 65 inches longin the direction of the axis from end wall 36 to end wall 38. Further,the thickness of adsorbent bed 40 is approximately 31/2 inches frominnerwall 44 to outer wall 46.

Inner wall 44 and outer wall 46 are both gas permeable and arepreferably constructed from a metal screen. Adsorbent bed inner wall 44defines cylindrically shaped inner air passageway 48. Air inlet port 30opens into inner passageway 48. Annularly shaped outer passageway 50 isformed between adsorbent bed outer wall 46 and adsorption unit outerwall 34. Adsorbent bed 40 may be supported within adsorption unit 16 bybolts 52 or other supporting means connected to endwall 36 and brackets54 or other supporting means generally disposed at the end of adsorbentbed 40 that is adjacent second endwall 38. Adsorption unit 16 may beconstructed, for example, from stainless steel using standard metalworking methods. The present invention is not limited to the structureshown in FIGS. 2-4. The multi-pass recirculation method of the presentinvention may also use other designs for adsorption unit 16.

In a preferred embodiment, air flows from fan outlet 22 through airinlet port 30 into inner passageway 48. Air then flows through adsorbentbed 40. As air flows through adsorbent bed 40, the air and thefumigation gas contained within the air contacts adsorbent material 42.The fumigation agent is then adsorbed by adsorbent material 42. Afterpassing through adsorbent bed outer wall 46, air then flows throughouter passageway 50. Finally, air flows along end wall 38 and out airoutlet port 32. The arrows shown in FIG. 3 generally show this air flowpattern. Ducts 14 and 18 may be connected to adsorption unit 16 and fan12 as shown in FIG. 1. In the embodiment shown in FIG. 1, duct 18 isconnected to air outlet port 32 and duct 14 connects to fan inlet 20.However, as discussed above various connection schemes may be used. Theair flow is not limited to the path shown in FIG. 3, and may flowopposite the direction shown. Thus, air may flow in air port 32 intoouter passageway 50. In this case, air flows through adsorbent bed 40into inner passageway 48 and out air port 30.

When removing hydrogen phosphide from the air, a preferred adsorbent isactivated carbon "DESOREX K-AG 03" available from Lurgi Aktivkohle GmbH.This adsorbent comprises a coal based carbon adsorbent containing 0.3%silver metal. The adsorbent adsorbs phosphine from the air, thencatalyzes its rapid oxidation to phosphates and lower oxidation statederivatives. This has the effect of increasing capacity of the materialover conventional fumigation adsorbents, since the adsorption sites arequickly freed up as phosphine is catalytically oxidized by air. Thepresent invention is not limited to this adsorbent, though, and othermaterials may be used.

In a preferred embodiment of the present invention, adsorption unit 16,as shown in FIGS. 2-4 and using Activated Carbon Desorex K-Ag 03, isarranged in a system such as shown in FIG. 1. A preferable air flow rateof approximately 900 cubic feet per minute (cfm) is used. However,generally air flow rates of 100 to 2000 cfm are contemplated andespecially air flow rates of 500 to 1000 cfm. Generally, fan powers of0.25 to 1.5 horsepower and air duct sizes of 4 to 12 inches arecontemplated and especially fan powers of 0.5 to 1.0 horsepower and airduct sizes of 6 to 10 inches. Thus, for example, with a 500,000 cubicfoot storage structure one air exchange may be completed inapproximately 9.25 hours. It is recognized, though, that multiplescrubber units may be affixed to a storage structure such that other airchange times for the structure may be achieved.

The contact time for a gas flowing through adsorbent bed 40 using thepreferred embodiment described above is generally 0.2 second, though,ranges from 0.05 to 0.6 seconds are contemplated and especially 0.1 to0.4 seconds. Such flow rates and contact times using the preferredadsorbent will result in an efficiency of approximately 80% to 90%hydrogen phosphide removal per pass. For most hydrogen phosphide removalpurposes, two air exchanges would thus provide generally sufficientfumigation gas removal of greater than 96%. It is recognized that foralternative fumigation gases, adsorbents, and adsorption units,different flow rates, efficiency rates, and contact times may bepreferred. Furthermore, the present invention is not limited to use withadsorbent materials, but rather may also be used with absorbentmaterials. However, the advantages of multi-pass recirculationfumigation gas removal would still be obtained.

The adsorption unit of the present invention has an advantage of beingportable. In the preferred embodiments described above, the unit maycontain only approximately 100 pounds of adsorbent and have a totalweight of approximately 200 pounds. Thus, the unit may be transportedfrom one storage structure to another. For example, the unit could beloaded in a truck or other vehicle for easy movement from site to site.

Further modifications and alternative embodiments of this invention willbe apparent to those skilled in the art in view of this description.Accordingly, this description is to be construed as illustrative onlyand is for the purpose of teaching those skilled in the art the mannerof carrying out the invention. It is to be understood that the forms ofthe invention herein shown and described are to be taken as thepresently preferred embodiments. Various changes may be made in theshape, size, and arrangement of parts. For example, equivalent elementsor materials may be substituted for those illustrated and describedherein, and certain features of the invention may be utilizedindependently of the use of other features, all as would be apparent toone skilled in the art after having the benefit of this description ofthe invention.

What is claimed is:
 1. A method of removing a gasiform fumigant from anagricultural storage structure, which comprises the steps of:providingan agricultural storage structure having a gasiform fumigant therein;providing a sorption unit connected to said storage structure, saidsorption unit containing a sorbent material capable of sorbing saidfumigant; forcing said fumigant from said structure through said sorbentmaterial; recirculating at least a portion of a gas output from saidsorption unit into said structure.
 2. The method of claim 1, whereinsaid fumigant is hydrogen phosphide.
 3. The method of claim 2, whereinsaid sorbent material is an adsorbent material and wherein an adsorptionefficiency during said forcing step is less than about 90% during afirst gaseous volume change of said structure.
 4. The method of claim 3,wherein said adsorbent material comprises an activated carbon andsilver.
 5. The method of claim 4, wherein said forcing step comprisesadsorbing phosphine, catalyzing said phosphine to phosphates and loweroxidation state derivatives.
 6. A method of removing a gasiform fumigantfrom an agricultural storage structure, which comprises the stepsof:providing an agricultural storage structure having a gasiformfumigant therein; providing a sorption unit connected to said storagestructure, said sorption unit containing a sorbent material capable ofsorbing said fumigant; passing gas from within said structure throughsaid sorption unit under conditions such that a single pass of said gasthrough said sorption unit is inadequate to reduce the concentration ofsaid fumigant in an output gas flow exiting said sorption unit to alevel below a predetermined level; and recirculating at least a portionof said output gas flow exiting the sorption unit through said structureand said sorption unit such that the concentration of said fumigant insaid output gas flow exiting said sorption unit is less than saidpredetermined level.
 7. The method of claim 6, wherein said gascomprises air.
 8. The method of claim 7, wherein:said fumigant compriseshydrogen phosphide; said sorption unit comprises an adsorption unitcontaining an adsorbent, said adsorbent comprising activated carbon andsilver.
 9. A method of fumigating an agricultural storage structure withhydrogen phosphide gas, which comprises the steps of:providing anagricultural storage structure; introducing gaseous hydrogen phosphideinto said structure; providing an adsorption unit connected to saidstorage structure, said adsorption unit containing an adsorbent materialcapable of adsorbing said hydrogen phosphide; forcing an input gasstream containing said hydrogen phosphide from said structure throughsaid adsorbent material; adsorbing phosphine from said input gas stream;recirculating at least a portion of an output gas stream from saidadsorption unit into said structure; and continuing said forcing,adsorbing and recirculating steps until the amount of said hydrogenphosphide within said structure is below a predetermined level.
 10. Themethod of claim 9, wherein an adsorption efficiency of said adsorbingstep is less than about 90% during a first gaseous volume change of saidstructure.
 11. The method of claim 9, wherein said forcing step isperformed at an input gas stream flow rate of less than about 2000 cfm.12. The method of claim 9, wherein said forcing step is performed at aflow rate such that said gas stream from said structure is in contactwith said adsorbent material contained in said adsorption unit for about0.05 to 0.6 seconds.
 13. The method of claim 9, wherein said forcingstep comprises:directing said input gas stream into a first passagewayof said adsorbent unit; and passing said hydrogen phosphide from saidfirst passageway through said adsorbent material into a secondpassageway of said adsorbent unit.
 14. The method of claim 13 whereinsaid adsorption unit comprises:an annularly shaped adsorbent bed forcontaining said adsorbent material, an inner wall of said adsorbent bed,said inner wall defining said first passageway, and an outer wall ofsaid adsorbent bed, said outer wall and a wall of said adsorption unitdefining said second passageway.
 15. The method of claim 14 wherein saidadsorption unit is portable.
 16. The method of claim 9 wherein saidadsorption unit comprises an annularly shaped adsorption bed, acylindrically shaped inner passageway and an annularly shaped outerpassageway.
 17. The method of claim 9 wherein said forcing stepcomprises:directing said input gas stream from said structure radiallythrough said adsorbent material between a central passageway within saidadsorbent material and an annular shaped outer passageway surroundingsaid adsorbent material.
 18. A method of reducing, to a predeterminedlevel, the concentration of a gasiform fumigant in a gas containedwithin an agricultural storage unit, which comprises:connecting anadsorption unit to a storage unit to form a flow loop for gas to flowthrough said adsorption unit and said storage unit, said adsorption unitcontaining an adsorbent for said fumigant in a quantity sufficient toreach said predetermined level upon passing the volume of gas in saidstorage unit through said adsorption unit more than once; andcirculating the volume of gas in said storage unit through said flowloop a sufficient number of times, greater than one, to reduce theconcentration of said fumigant in said gas to said predetermined level.